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Addendum to the Well Installation and Hydraulic Testing Report and Additional ARCADiS Well Installation Work Plan Former Occidental Chemical Corporation Lathrop, California Well Name Well Type Aquifer Zone Well Depth Screen (ft bgs) Interval` (ft b s INJ-12 Injection Shallow 100 70-100 Intermediate 150 120-150 INJ-12 Injection Deep 200 170-200 INJ-13/EW-03 Injection Shallow 100 70-100 Intermediate 150 120-150 INJ-14/EW-04 Injection Intermediate 150 120-150 Deep 200 170-200 Notes: Well casings will be 6-inch diameter steel "=Extraction/Injection well screen will be 0.010-inch factory-cut slotted steel and wire wrapped 1 =Well installation is contingent upon further evaluation of COC concentrations in groundwater 2=Well to be installed in the future based on system performance 3=Well installation is contingent on the conversion of existing wells for injection purposes A summary of proposed extraction and injection wells within each zone is provided in the following sections. Proposed Site-specific waste discharge requirements(WDR) and associated monitoring and reporting requirements for the expanded system are currently being reviewed by the RWQCB. Water Table Zone(0 to 50 ft bgs) Within the water table zone, three extraction wells are proposed to address residual sulfolane within the sulfolane investigation and treatment area. Prior to the installation and startup of EW-09 on July 1, 2012, groundwater was not previously extracted from the water table zone as part of the Site remedy. Figure 7 shows the extraction well locations relative to the approximate extent of sulfolane, DBCP, and EDB. Extraction wells EW-10 and EW-11 are proposed to be placed approximately 80 to 120 ft apart, near EW-09,within the sulfolane area.The third extraction well EW-17 is proposed to be placed near the PW22 monitoring well cluster, approximately 1,200 ft southwest of EW-11, primarily to address DBCP concentrations up to 2,200 pg/L and sulfolane concentrations up to approximately 170 pg/L. Based on historical data from water table zone monitoring wells,the sulfolane plume has shrunk significantly since 1982 to the current plume footprint. Monitoring wells PW02-040, PW03-049, PW07- 038, and PW13-046 all had sulfolane detections historically, while recent groundwater monitoring results show that sulfolane is currently non-detect in all of these wells. Recently, Simplot shared sulfolane concentration data with GSH from a July 2012 sampling event. Sulfolane was detected in water table zone wells LP-1 (200 pg/L) and LP-10 (20 pg/L). Sulfolane was detected in PWB-1 boring (170 pg/L) in the Ag Chem Area. Sulfolane was historically detected in several exploratory borings advanced by Simplot in 1995.The current plume footprint interpretation shown on Figure 7 and the updated groundwater modeling runs are consistent with these results. 2012-09-13 addendum well install work plan_hy test rpt.docx 11